1. Field of the Invention
The present invention generally relates to a method and compound for the treatment of cancer. More particularly, an embodiment relates to the use of DNA interactive compounds that bind to DNA and undergo a series of chemical reactions in the presence of DNA to generate reactive intermediates that cleave DNA.
2. Brief Description of the Related Art
In 1972 Robert Bergman and co-workers demonstrated the gas-phase thermal rearrangement of substituted 3-hexene-1,5-diynes (1A/1B, FIG. 1), and proposed the intermediacy of a 1,4-5 didehydrobenzene, 2 in this process (Jones and Bergman, 1972). Indirect evidence for the existence of a singlet 1,4-didehydrobenzene intermediate was provided by solution-phase CIDNP experiments, which afforded the substituted benzene products 3 (Lockhart and Bergman, 1981). Bergman's original finding has gained additional significance in light of the discovery of an entire class of antitumor antibiotics, exemplified by calicheamicin .gamma..sub.1.sup.I (4, FIG. 2) (Lee, 1987) that exert their potent cytotoxic effects through a Bergman cyclization of an enediyne core to produce a 1,4-didehydrobenzene intermediate. This diradical abstracts hydrogen atoms from the DNA ribose backbone, resulting in DNA strand scission (Hangeland, 1992).
Although simple, acyclic enediynes generally require higher temperatures than is physiologically relevant for Bergman cyclization to take place, synthetic enediynes that are strained may cyclize and produce DNA cleaving diradicals under physiological conditions, (Nicolaou, Dai, Tsay, Estevez, and Wrasidlo, 1992) and large numbers of these reactive enediynes have been designed, synthesized, and evaluated for biological activity (Grissom, Gunawardena, Klingberg, and Huang, 1996). More recently, the synthetic utility of the Bergman cyclization has been explored, principally by Grissom, who has employed the 1,4-didehydrobenzene intermediates afforded by the Bergman cyclization of substituted 3-hexene-1,5-diynes and substituted 1,2-diethynylbenzenes in subsequent free radical reactions to rapidly construct polycyclic compounds (Grissom, Calkins, Huang, and McMillen, 1994).
A related diradical-generating cyclization of 1,2,4-triene-5-ynes, modeled on the presumed DNA strand scission chemistry of the neocarzinostatin chromophore (Edo, 1985) (5, FIG. 3), has been discovered by Myers and co-workers (Myers, 1989). These workers found that enyne allene 6 undergoes an exothermic conversion to the .alpha.,3-didehydrotoluene intermediate 7, which may either abstract hydrogen atoms from 1,4-cyclohexadiene to produce toluene (8) or combine with the cyclohexyldienyl radical to form the adduct 9 (FIG. 4). This Myers cyclization has been exploited by many workers in the design of simple diradical-generating compounds with demonstrable ability to cleave DNA under physiological conditions (Nicolaou, Maligres, Shin and Deleon, 1990). The Myers cyclization has also been employed synthetically by Grissom (Grissom, Klingberg, Huang, and Slattery, 1997) and Wang (Wang, Wang, Tarli, and Gannet, 1996) in the construction of polycyclic molecules.
Schmittel and co-workers, (Schmittel, et al., 1995) and others (Gillman, et al., 1995) have reported anomalous products 12, 13, and 14 of thermal cyclizations of enyne allenes 10 (FIG. 5). These products are more pronounced in cases where the enyne allene substituents R, R.sup.1, or R.sup.2 are large. In these cases, the enyne allenes undergo cyclization to the benzofulvalene biradical intermediate 11, the fate of which is dependent upon the nature of the substituents. Schmittel has demonstrated that enyne allenes that undergo this C.sup.2 -C.sup.6 cyclization reaction are able to cleave DNA, presumably as a result of hydrogen atom abstraction by the diradical 11 (Schmittel, Maywald, and Strittmatter, 1997).
Despite the promise, both synthetic and biological, of the chemistry of enediynes and enyne allenes, heteroatom substituted variants of these systems have not been extensively explored. Moore (Moore, 1992) has found that the enyne ketenes 16, generated from thermolysis of cyclobutenones 15, afford quinones 18, through the intermediate diradicals 17 (FIG. 6). These cyclobutenones also exhibit DNA cleaving ability, presumably due to the ability of the diradical intermediates to abstract hydrogen atoms from the DNA backbone (Sullivan, 1994). Padwa (Padwa, 1993) and Nakatani (Nakatani, 1994) have used alternative routes to enyne ketenes, which were also found to afford cycloaromatized products through diradical intermediates.
In contrast to the oxo-substituted enyne allene system, few aza-substituted enediyne or enyne allenes had been reported prior to our work. Wang and co-workers had reported the failed attempt to coax nitrile (19, FIG. 7) to undergo an aza-Myers cyclization (Wang, Wang, and Sattsangi, 1996). Gillman and co-workers had reported similar findings for a related 2-allenyl cyanobenzene (Gillman and Heckhoff, 1996). Most recently, Wang and co-workers have shown that the ketenimine 20 gives products predicted by both an aza-Myers cyclization (21) and the C.sub.2 -C.sup.6 cyclization (22) (FIG. 8) (Shi and Wang, 1998).